ISDN BRI

ISDN BRIs are common in branch and telecommuter installations in which higher than asynchronous
bandwidth is needed. The BRI specification avails two 64Kbps bearer channels (B channels)
for user traffic, and it uses a single 16Kbps D channel for management and signaling. It is important
to remember that these connections are circuit switched and that the data link protocol on the
D channel is Link Access Procedure, Data (LAPD).
This differs from the X.25 protocol, which uses LAPB. The ISDN B channel is similar to
a standard voice channel in terms of bandwidth, and therefore most systems allow the use
of a B channel for a traditional analog call. Although the single channel is encoded digitally
from the ISDN device to the switch—unlike an analog connection from a phone to a phone
switch—the overall mechanics between them are similar.
The ISDN BRI is terminated with different connections, but the network (phone company)
is usually terminated with an RJ-11 or RJ-45 interface. According to the specifications, the termination
should always be accomplished with an RJ-45, which provides for additional signaling
and visually distinguishes the ISDN interface from analog connections. However, the
exterior pins (1, 2, 7, and 8) of the RJ-45 are frequently unused, so some providers use RJ-11
instead. If you can control this part of the installation, specify RJ-45 and use a specific color to
differentiate it from Ethernet, T-1, and other connections.

Note:Some installations of ISDN allow only 56Kbps for each B channel. The reference
to X.25 is incorporated in this section due to the comparison provided by
the protocol. X.25 is a highly robust protocol, but is not commonly deployed
and is no longer part of the exam.